Process for the production of magnesium hydrides

ABSTRACT

A process for the production of hydrides of magnesium, comprising reacting magnesium with hydrogen in the presence of magnesium.

This application is a continuation-in-part of application Ser. No.433,078, filed Oct. 6, 1982, which is a continuation-in-part ofapplication Ser. No. 187,907, filed Sept. 17, 1980, which is acontinuation of application Ser. No. 008,739, filed Feb. 2, 1979, allnow abandoned.

The invention relates to a catalytic process for the preparation ofmagnesium hydrides under mild conditions. For the preparation ofmagnesium hydride by prior art methods, high reaction temperatures andpressures are generally required: 175° C./5 bar [in the presence ofallyl iodide or iodine; J. P. Faust et al, J. Appl. Chem. (London) 10,187 (1960)]; 140° to 170° C./135 bar [in the presence of AlEt₃ ; M.Mamula et al, Czech Pat. No. 139,589 (1966), C.A. 76, 87972 (1972)];150° C./200 bar [J. C. Snyder, U.S. Pat. No. 3,485,585 (1964), C.A. 72,45603 (1970); TiCl₃ as catalyst; J. C. Synder, U.S. Pat. No. 3,387,948(1962), C.A. 69, 44943 (1968)]; 350° C./24 bar (J. J. Reilly and R. H.Wiswall, Inorg. Chem., 2254, 1968); and 380° to 450° C./100 to 200 bar[T. N. Dymova et al, Zhur. Neorg. Khim. 6, 763 (1961), C.A. 55 23144(1961)].

The formation of MgH₂ from magnesiun and H₂ in the presence of VCl₄ ascatalyst in tetrahydrofuran at 20° C./1 bar has been described by B.Jezowska-Trzebiatowska et al. [Bull, Acad. Pol. Sci., Ser. Sci. Chim.1976, 24 (4), 331; C.A. 85, 180094 (1976)] However, the catalyticactivity of this system drops off so markedly even after a few catalyticsteps that industrial production of MgH₂ by this route is impractical.According to E. C. Ashby and A. B. Goel [J. Amer. Chem. Soc. 99, 310(1977)], halomagnesium hydrides soluble in THF could heretofore beproduced only by reacting a magnesium hydride obtained from dialkyl ordiaryl magnesium and LiAlH₄ with magnesium halides.

For the production of magnesium hydrides by the process in accordancewith the invention, metallic magnesium is reacted with hydrogen in thepresence of a halide of a transition metal of the fourth to eighthsub-group of the periodic system and an organo-magnesium compound ormagnesium hydride or a polycyclic aromatic or a tertiary amine and amagnesium halide. Preferred transition metal compounds are chromium,iron and titanium halides.

The magnesium halide can be employed in catalytic amount relative tomagnesium, in which event the magnesium product will be substantially"pure" magnesium hydride. If larger amounts of magnesium halide areemployed, the resulting magnesium hydride will include halide insubstantial amount, probably present as halo-magnesium hydrides. If themagnesium halide is present in stoichiometric amount relative to themagnesium to be hydrogenated, substantially each molecule of productwill contain halide. Beyond stoichiometric amounts of magnesiumhalide:magnesium, the product will contain molecules of magnesiumhalides which contain no hydrogen, clearly pointless. Anthracene per seis the preferred polycyclic aromatic but other polycyclic aromatics suchas the fused anthracenes, tetracene and pentacene are also suitable.

Other activators may optionally also be present, such as alkyl,cycloalkyl and/or aryl tertiary amines, e.g. triethyl amine, tributylamine, pyridine, and the like. Alkyl halides such as ethyl bromide,which form Grignard reagents with the magnesium, are also usefulactivators and speed up the process. Advantageously, the alkyl radicalshave up to 10 carbon atoms and preferably up to 4 carbon atoms, thecycloalkyl radical up to 10 carbon atoms, and the aryl radical ispreferably phenyl or naphthyl.

In accordance with the invention, the magnesium halide, the transitionmetal halide, the anthracene and the activator, if present, may be usedin catalytic quantities, e.g. about 1:10-1:10⁴, the latter havingreference to Mg. However, if desired, these components can be present inmuch larger proportions relative to Mg, e.g., magnesium halides can beemployed in stoichiometric amount (or even more) relative to Mg.

The operating temperature may range from 0° to 200° C., advantageously0° to 150° C., and preferably room temperature to 80° C., at from about1 to 300 bar, preferably about 40 to 140 bar.

The hydrogen gas may be substantially pure or it may be present inadmixture with other gases which are inert under the prevailingconditions, e.g., nitrogen, methane, homologs of methane, noble gases,and the like, or which are moderate reactive and present in smallamounts under the prevailing conditions, e.g., carbon monoxide, carbondioxide, oxygen, water vapor, ammonia, and hydrogen sulfide, and thelike. In the latter case, part of the hydrogen capacity of the activemagnesium is reduced.

The process in accordance with the invention makes possible for thefirst time the industrial production of highly reactive magnesiumhydrides including soluble halomagnesium hydrides from magnesium andhydrogen under mild conditions. Magnesium hydride is of great importanceas a hydrogen store since it contains 7.66 wt. % of hydrogen which canbe split off at elevated temperatures with reverse formation ofelemental magnesium. [See D. L. Cummings and G. J. Powers, Ind. Eng.Chem., Process Des. Develop. 13, 182 (1974)].

While hydrogenation of the magnesium can be carried to completion, viz.all of the magnesium is converted to MgH₂, it can be terminated soonerand it is another advantage of the invention that more than 50% of themagnesium can be converted to the hydride relatively rapidly.

The magnesium hydrides produced by the process in accordance with theinvention as a hydrogen store are superior to magnesium hydridesproduced by prior art methods. [See also M. H. Mintz et al, J. Inorg.Nucl. Chem. 40, 765 (1978)]. Thus a sample (15.8 g) of the magnesiumhydride prepared by the use of CrCl₃ as a catalyst component at 20° C.was quantitatively dehydrogenated within 40 min. at 300° to 315° C. and1 mm Hg. (Release of 11.5 l H₂ at 1 bar and 20° C.) The highly activemagnesium powder so obtained again reacts with hydrogen already undersmall partial pressure of hydrogen at relative mild conditions oftemperature and pressure, below 300° C. or even below 150° C. and below50 bar or even at 1-3 bar, the formation of magnesium hydride beingpractically completed within 30 min. (Absorption of 11.5 l H₂ at 1 barand 20° C.) The hydrogenation/dehydrogenation process may be repeatedseveral times without appreciable variations occurring in the rate or inthe H₂ storage capacity, or in the high quality of the pure hydrogencoming out of the dehydrogenation step. Purification of hydrogen out ofgas mixtures is a further application of the process.

Moreover, magnesium hydride per se or the soluble halomagnesium hydridesmay be used in place of the costly LiAlH₄ as reducing agents. They mayalso be used in the synthesis of magnesium alkyls and hydrides of othermetals, in the preparation of "activated" magnesium, as a dehydratingagent, a carrier for heterogeneous catalysts, and the like.

It is especially useful to effect the hydrogenation in the presence of amagnesium halide in conjunction with a transition metal hadide,especially a hadide of titanium, chromium and/or iron. The presence ofmagnesium halide shortens the hydrogenation time by a factor of 5 to 10.Therefore, production costs in the preparation of magnesium hydride areextremely lowered and the volume/time yield raised respectively. As anindex for the increase of activity besides the raised yield the activesurface of the produced magnesium hydride is increased.

The catalysts can be employed in accordance with the invention in aratio of magnesium to transition metal of 10⁴ -50:1, whereby a ratio oftransition metal to magnesium-organic compound or magnesium hydride tomagnesium halide of 0.1:1:2 to 10:1:20 can be selected. When, however,the operation is carried out at a high molar ratio of magnesium totransition metal (e.g. 5×10² :1), a molar ratio of magnesium halide totransition metal that is even higher than 20:1 (like 50:1 or 100:1 e.g.)can also be selected.

European Pat. No. 0 003 564 employs magnesium halide in stoichiometricamount relative to magnesium for hydrogenation. Examples 35 to 42hereinbelow illustrate the advantages of only catalytic quantities ofmagnesium halide.

It takes 1.5 to 2 hours (Example 35) to quantitatively hydrogenate50-mesh magnesium powder by the instant method at 60° to 64° C. and ahydrogen pressure of 60 bars with a molar ratio of magnesium to titaniumto anthracene to MgCl₂ of 100:1:1:8.7 (for a titanium or anthraceneconcentration of 0.05 moles/l). By the method described in European Pat.No. 0 003 564, however, subject to the same conditions but without theaddition of catalytic amounts of MgCl₂ (comparative test, Example 35),it takes about 20 hours to completely hydrogenate the magnesium. Whenthe magnesium is hydrogenated by the instant method subject to theconditions described in Example 35 but employing 270-mesh magnesiumpowder, hydrogenating time is decreased to 45 minutes. Increasinghydrogen pressure from 60 to 120 bars decreases the hydrogenating timeof 50-mesh magnesium powder from 1.5-2 hours to 1 hour (Example 35 or37). As comparison of Examples 35 and 38 indicates, the rate ofhydrogenation increases as temperature is increased from 40° to 60° C.

The properties of the magnesium hydride obtained by the instant methodare definitely better than those of the magnesium hydride obtained bythe method described in European Pat. No. 0 003 564. As measured by BET,the specific surface of the magnesium hydride prepared as described inExample 35 or 36 herein is 129.4 m² /g as against 70 to 90 m² /g for themagnesium hydride obtained subject to the same conditions but withoutthe addition of magnesium halides. The magnesium hydride prepared by theinstant method will accordingly be preferred for chemical purposes(synthesizing magnesium alkyls and metal or element hydrides, preparing"active magnesium," reducing or drying agents, and carriers forheterogeneous catalysts) and to store, purify, or separate hydrogen.

All the tests described in the examples were carried out in a protectiveargon atmosphere.

EXAMPLE 1

(a) A suspension of 97.2 g (4.0 moles) of magnesium powder (50 mesh) in400 ml abs. THF is mixed with 1.0 ml of ethyl bromide, and after half anhour's stirring with 8.0 g (45 mmoles) of anthracene. After 3 hours'stirring of the mixture (formation of magnesiumanthracene), 7.0 g (44mmoles) of CrCl₃ is added, stirring then being continued for 25 to 30min. The olive-green suspension is then hydrogenated in a 2-literautoclave with agitator at 52° C. and an initial pressure of 135 barshydrogen. After a reaction time of 5 hr., the hydrogen pressure is 92bars, and after 8 hr., 82 bars; and after a total reaction time of 20hr. the pressure remains constant at 72 bars. The pressure losscorresponds to an uptake of 100 l of hydrogen, or a quantitativeconversion of Mg to MgH₂. The magnesium hydride may be separated fromthe catalyst solution by filtration of the suspension obtained, and maythen be obtained in solid, pyrophoric form by washing with THF andpentane or drying under vacuum. Yield: Quantitative.

(b) To show its beneficial effects, the activator ethyl bromide isomitted, the process being re-run as follows:

A suspension of 97.2 g (4.0 moles) of magnesium powder (50 mesh) in 400ml abs. THF is mixed with 8.0 g (45 mmoles) of anthracene. After 10-12hours' stitting of the mixture (formation of magnesium anthracene) 7.0 g(44 mmoles) of CrCl₃ are added, stirring then being continued for 30min. The olive-green suspension is then hydrogenated as described in(a) 1. The absorption of hydrogen up to the complete hydrogenation ofthe magnesium develops as described there.

It takes much longer to form the magnesium anthracene which is anessential ingredient during hydrogenation. Once it is formed thehydrogenation after the addition of the transition metal halide isunchanged as far as speed and yield are concerned.

The time of formation of the magnesium anthracene without the additionof ethyl bromide or iodine can be reduced below the three hoursdescribed in (a), if the magnesium powder is heated in vacuum (0.1 mbar)before the addition of THF.

EXAMPLE 2

97.2 g (4.0 moles) of magnesium powder in 250 ml of abs. THF isintroduced into a 2-liter autoclave with agitator and activated by theaddition of 0.4 g of iodine. A suspension of 3.8 g (19 mmoles) ofmagnesiumanthracene and 0.5 g (20 mmoles) of magnesium powder in 180 mlTHF is mixed with a solution of 3.0 g (17.5 mmoles) FeCl₃ in 20 ml THF,the mixture is stirred for 20 min. and then added to the contents of theautoclave. The mixture is hydrogenated at 52° C. and an initial pressureof 120 bars hydrogen. After a reaction time of 24 hr., the hydrogenpressure is 80 bars, and after 48 hr. the hydrogen pressure remainsconstant at 62 bars. The pressure drop corresponds to a total uptake of4.3 moles of H₂, or complete conversion of the Mg to MgH₂.

EXAMPLE 3

A suspension of 4.86 g (0.2 mole) of magnesium powder (50 mesh) in 50 mlabs. THF is mixed with 0.05 ml of ethyl bromide, and after half anhour's stirring with 0.36 g (2.0 mmoles) of anthracene. After 3 hours'stirring of the mixture (during which time magnesiumanthracene isformed, as is apparent from the separation of the orange precipitate),2.0 mmoles of the particular transition-metal compound (see Table) isadded to the suspension, followed by stirring for another 15 to 20 min.The reaction vessel is filled with hydrogen at normal pressure and thehydrogen uptake is measured at 20° C. with vigorous stirring by means ofa gas burette. The hydrogen uptake proceeds at a nearly constant rateover several days. The data on the amounts of H₂ taken up after 48 hr.and on the percent conversion to MgH₂ are presented in the Table.

                                      TABLE                                       __________________________________________________________________________    Example                                                                             3   4   5    6   7  8   9   10  11                                      __________________________________________________________________________    Transition-                                                                         CrCl.sub.3                                                                        FeCl.sub.3                                                                        FeAcac.sub.3 *                                                                     TiCl.sub.4                                                                        VCl.sub.4                                                                        MoCl.sub.5                                                                        MnCl.sub.2                                                                        CoCl.sub.2                                                                        NiCl.sub.2                              metal                                                                         compound                                                                      g (2.0 mM)                                                                          0.32                                                                              0.32                                                                              0.71 0.38                                                                              0.39                                                                             0.56                                                                              0.25                                                                              0.26                                                                              0.26                                    H.sub.2 uptake                                                                      2000                                                                              1500                                                                              300  1300                                                                              500                                                                              700 950 250 200                                     after 48                                                                      hr. (ml)                                                                      Percent                                                                              42  31  6    27  10                                                                               15  20  5   4                                      converted                                                                     to MgH.sub.2                                                                  after 48 hr.                                                                  __________________________________________________________________________     *Acac = acetyl acetonate                                                 

EXAMPLES 12-15

Examples 12-15 are carried out analogously to Example 4, the molar ratioof FeCl₃ to magnesiumanthracene being varied from 1:0.5 to 1:10. Thefigures for the amounts of H₂ taken up after 48 hr. and for the percentconverted to MgH₂ are presented in the Table which follows.

    ______________________________________                                        Example    12       4       13     14    15                                   ______________________________________                                        FeCl.sub.3 g (mM)                                                                        0.32 (2.0)                                                                             0.32    0.32   0.32   0.32                                Anthracene g                                                                              0.18    0.36    0.71   1.42  3.6                                  millimoles (1.0)    (2.0)   (4.0)  (8.0) (20.0)                               H.sub.2 uptake                                                                           850      1,500   850    900   1,300                                after 48 hr.                                                                  (ml)                                                                          Percent     18        31     18     19     27                                 converted                                                                     to MgH.sub.2                                                                  after 48 hr.                                                                  ______________________________________                                    

EXAMPLES 16-18

Examples 16-18 are carried out analogously to Example 4, the mixturesbeing mixed with 10 millimoles of an amine (see Table) prior tointroduction of the hydrogen. The figures for the amounts of H₂ taken upafter 48 hr. and for the percent converted to MgH₂ are set forth in thefollowing table:

    ______________________________________                                        Example 1617                  18                                              ______________________________________                                        Amine   (C.sub.2 H.sub.5).sub.3 N(CH.sub.3).sub.2 NCH.sub.2 CH.sub.2                  N(CH.sub.3).sub.2                                                                                    ##STR1##                                       g (milli-                                                                             1.01 (10.1)1.16 (10.0)                                                                              1.12 (10.0)                                     moles)                                                                        H.sub.2 uptake                                                                        2,0001,100            700                                             after 48 hr.                                                                  (ml)                                                                          Percent   42  23               15                                             converted                                                                     to MgH.sub.2                                                                  after 48 hr.                                                                  ______________________________________                                    

EXAMPLE 19

This experiment is carried out analogously to Example 4, a mixture of 40ml of toluene and 10 ml of tetrahydrofuran being used as solvent. Thehydrogen uptake after 48 hr.

EXAMPLE 20

This experiment is carried out analogously to Example 4,1,2-dimethoxyethane (50 ml) being used as solvent in place oftetrahydrofuran. The hydrogen takeup after 48 hr. was 220 ml.

EXAMPLES 21-27

Examples 21-27 are carried out analogously to Example 4, theorganometallic compounds (M-C) (2.0 millimoles) given in the table beingused as catalyst component in place of magnesiumanthracene. The figureson the hydrogen uptake after 48 hr. are presented in the table:

    __________________________________________________________________________    Example                                                                              21  22   23  24    25 26   27                                          __________________________________________________________________________    M--C   AlEt.sub.3                                                                        BEt.sub.3                                                                          ZnEt.sub.2                                                                        NaC.sub.6 H.sub.5                                                                   LiBu                                                                             Na-- Et.sub.2 AlCl                                                            naphth.                                          g (2.0 0.23                                                                              0.20 0.25                                                                              0.20  0.13                                                                             0.30 0.26                                        mM in                                                                         each case)                                                                    H.sub.2 uptake                                                                       550 520  570 400   570                                                                              450  300                                         after 48                                                                      hr. (ml)                                                                      __________________________________________________________________________

EXAMPLE 28

A suspension of 0.60 g (25 millimoles) of magnesium powder in 10 ml ofabs. THF is mixed with 0.02 ml of ethyl bromide, and after half anhour's stirring with 0.06 g (0.37 millimoles) of anthracene. After 3hours' stirring, 0.06 g (0.37 millimoles) of CrCl₃ is added to thesuspension, and after another 15 min. a solution of 4.9 g (27millimoles) of MgBr₂ (anhydrous) in 120 ml of THF. The mixture ishydrogenated in a glass-lined 500-ml autoclave with agitator for 12 hr.,15° C. and 100 bars hydrogen pressure. The suspension is allowed tosettle for 20 hr. Upon deuterolysis, 25.0 ml (out of a total of 130 ml)of the clear supernatant solution yielded 140 ml HD. The solutionfurther contained a total of 1.15 g (47.4 millimoles) of Mg and 4.28 g(53.6 mg-atom) of Br. This composition corresponds to a yield of solubleHMgBr (in mixture with MgBr₂) of about 60%.

EXAMPLE 29

97.0 g (4.0 moles) of magnesium powder in 370 ml of abs. THF isintroduced into a 2-liter autoclave with agitator and activated by theaddition of 1.0 ml of ethylbromide. 2 liters (83 millimoles) of gaseousbutadiene is then introduced into the suspension, and the contents ofthe autoclave are then heated over 1.5 hr. to 80° C. (formation of anorganomagnesium compound). After cooling to room temperature, 3.2 g (19millimoles) of FeCl₃ in 30 ml of THF is added to the contents of theautoclave. The mixture is hydrogenated at 20° to 22° C. and an initialhydrogen pressure of 120 bars. After a reaction time of 20 hr. thehydrogen pressure drops to 97 bars, and after 44 hrs. the pressureremains constant at 92 bars. The pressure drop corresponds to a totaluptake of 2 moles of H₂, or a conversion of Mg to MgH₂ of about 50%.

EXAMPLE 30

This experiment is carried out analogously to Example 3, (π-C₅ H₅)₂ Cr(0.36 g=2.0 millimoles) being used as catalyst component in place ofCrCl₃. The hydrogen uptake after 48 hr. is 850 ml.

EXAMPLE 31

15.2 g MgH₂ prepared according to Example 1 were dehydrogenated at 372°C. and normal pressure using a 300 ml stainless steel autoclave.Thereafter at 227° C. the autoclave was evacuated, and brought to apressure of 10 bar with an 85.8-14.2 volume % CH₄ /H₂ -mixture. A shortincrease in temperature to 231° C. was noticed. After 1.2 hours at 227°C. the autoclave was depressurized and the released gas analyzed: 1.9Vol.% H₂, 98.1 Vol.-% CH₄. This hydrogen content corresponds almost tothe calculated equilibrium vapor pressure of the hydrogen abovemagnesium hydride at 227° C. (calculated 1.5 Vol.-% H₂).

Hydrogenation of the active magnesium using the CH₄ /H₂ -mixture wasconducted in 36 steps in order to demonstrate the capacity of the activemagnesium. The temperature was kept constant, pressure and contact timewere varied according to the data of the following table, the autoclavebeing de-pressurized and the residual gas being analyzed after eachstep:

    ______________________________________                                        Hydrogenation                                                                            Pressure Hydrogenation                                                                              Mole % H.sub.2 in                            Step       (bar)    Time (h)     residual gas                                 ______________________________________                                        2.         10       2.4          2.6                                          3.         10       0.33         4.1                                          4.         10       12.0         2.4                                          5.         15       1.6          1.9                                          6.         15       0.33         2.6                                          7.         15       4.0          1.4                                          8.         15       12.0         1.2                                          9.         15       1.5          2.1                                          10.        15       1.5          2.3                                          11.        15       1.5          2.2                                          22.        15       1.5          9.8                                          37.        15       1.5          12.5                                         ______________________________________                                    

After cooling the autoclave to room temperature and evacuating to 0.1millibar the autoclave was heated to 363° C. whereby during 1 hour 9.32l gas (20° C./1 bar) were collected. After the start and before the endof the gas development a gas sample was analyzed. Each analysisconfirmed 100% H₂. From the amount of H₂ released it was calculated that66% of the magnesium had been converted to MgH₂.

EXAMPLE 32

Repeating Example 31, but using a hydrogenation temperature of 196° C.,a pressure of 15 bars and 14.3 g of the original MgH₂, the magnesium washydrogenated in 23 independent steps, each lasting 2 hours. The Hcontent of the residual gas can be seen from the following table:

    ______________________________________                                        Hydrogenation  Mole % H.sub.2 in                                              Step           residual gas                                                   ______________________________________                                         1.            0.7                                                             2.            0.6                                                            11.            1.4                                                            13.            2.7                                                            23.            12.6                                                           ______________________________________                                    

After dehydrogenation at 362° C. there were collected 8.46 l gas (20°C./1 bar) consisting of 100% H₂. This corresponded to 60% of themagnesium having been converted to MgH₂.

EXAMPLE 33

17 g of a MgH₂ prepared according to Example 1 were dehydrogenated at330° C./normal pressure (11 l H₂ at 20° C.) to form an active magnesium.

Thereafter hydrogen containing 1 mol% CO were pressed on the activemagnesium up to a pressure of 10 bar, keeping this pressure during thehydrogenation. The temperature varied between 373° and 330° C. during 70minutes. 12.5 l (20° C./1 bar) hydrogen were absorbed. Cooling theautoclave to room temperature and releasing the residual pressure thegas above the formed MgH₂ contained 89 mol% H₂ and 8 mol% CH₄. Theamount of methane formed corresponds to the amount of carbon monoxidepresent in the absorbed amount of H₂. Dehydrogenation at 0.2 mbar and338° C. during 70 min. 13.3 l (20° C., 1 bar) of pure hydrogen areformed.

Hydrogenation with a hydrogen/CO-gas mixture of the active magnesium anddehydrogenation as described were repeated 27 times. Duringhydrogenation the temperature went up to 370°-373° C. while duringdehydrogenation the temperature decreased to about 300° C. After the7th, 13th, 25th, 27th dehydrogenation the respective amounts of gasreleased were 11.3, 10.5, 8.8, and 8.5 l (20° C., 1 bar) of H₂. The gasafter the 27th dehydrogenation step was still pure hydrogen. After thisnumber of cycles the capacity of the MgH₂ battery was reduced to 70%compared with the original sample.

EXAMPLE 34

16 g of magnesium hydride prepared according to Example 1 weredehydrogenated in a 300 ml stainless steel autoclave in a vacuum at 0.2mbar by slowly raising the temperature of the autoclave (about 2° C. perminute) to 373° C. At about 270° C. the evolution of gas starts. 12.8 lgas (20° C., 1 bar) were developed. The active magnesium so produced washydrogenate at 338° C. and 10 bar by a H₂ /CO-gas mixture containing 2.3mol% CO. Keeping the pressure constant, temperature shortly went up to366° C. and dropped thereafter during 1.5 hours back to 338° C. Theresidual gas in the autoclave (11.2 l, 20° C., 1 bar) contained 82.9mol% H₂, 15.8 mol% CH₄, rest N₂. Thereafter the autoclave was cooled toroom temperature evacuated and thereafter the formed MgH₂ at 340°C./normal pressure dehydrogenated. 11.2 l (20°, 1 bar) of pure H₂ wereobtained.

Repeating hydrogenation (hydrogen containing 2.3 mol% CO) anddehydrogenation further 25 times after the 8th, 12th, and 26thdehydrogenation 5.9, 4.9, and 2.4 l (20° C. and 1 bar) respectivelyhydrogen were formed.

EXAMPLE 35

48.6 g (2.0 moles) of magnesium powder (50 mesh) and 3.56 g (20.0mmoles) of anthracene were placed in a 1-1 flask and 370 ml of a 0.47molar MgCl₂ solution in absolute THF (tetrahydrofuran; 174 mmoles;anthracene:MgCl₂ =1:8.7; MgCl₂ solution prepared from magnesium powderand 1,2-dichloroethane in THF) and 0.3 ml of ethyl bromide added. After5 to 10 minutes of stirring the suspension took on a deep-bluecoloration. After 15 minutes of stirring the suspension was treated with6.7 g (20.0 mmoles) of TiCl₄ 2 THF, whereupon heat tonality and a colorchange toward dark brown occurred, and stirred for another 15 minutes atroom temperature. The suspension was poured into a 1 liter special-steelautoclave equipped with a blade agitator, thermosensor, andthermostating device, the contents heated to 45° C., and hydrogen pumpedin from a pressurized supply container through a pressure-reducing valveuntil the pressure was 60 bars. The exterior autoclave temperature wasincreased to 60° C. and the contents of the autoclave isobaricallyhydrogenated at a stirring speed of 900 min⁻¹ and a hydrogen pressure of60 bar. The temperature of the reaction mixture and the hydrogen uptake(as mentioned by the pressure drop in the supply container) was graphedduring the test on a two-channel recorder. The temperature of thereaction mixture increased temporarily to 64° C. as hydrogenationcommenced and decreased to 60° C. as hydrogenation continued. After 1.5to 2 hours the hydrogenation process was terminated and hydrogen uptakequantitative. The suspension of MgH₂ was filtered, the filter cakewashed twice with THF and twice with pentane, and the magnesium hydridedried in a high vacuum at 20° C.

Yield: 45.0 g of MgH₂.

Composition: Mg 82.5, H 7.1, C 2.8, Cl 2.2, Ti 0.5%.

Specific powder surface (BET): 129.4 m² /g.

It should be noted that the filtrate can be employed with the dissolvedcatalyst (without washing liquid) to hydrogenate another charge ofmagnesium with the same results. Subsequent to two hydrogenations, thehydrogenating action of the catalyst solution will of course becomeweak.

A comparative test of the hydrogenation of magnesium was conductedanalogously and with the same amounts of materials but without theaddition of MgCl₂ (as known from European Pat. No. 0 003 564).Hydrogenation took 20 hours.

EXAMPLE 36

The test was conducted as in Example 35 and with the same amounts ofmaterial but with 270-mesh magnesium powder. The hydrogenation processwas terminated after 45 minutes.

Yield: 44.8 g of magnesium hydride.

Comp.: Mg 81.8, H 7.3, C 5.2, Cl. 4.5, Ti 1.0%.

Specific surface: 129.4 m² /g.

EXAMPLE 37

The test was conducted as in Example 35 and with the same amounts ofmaterial but at a hydrogen pressure of 120 bar. Hydrogenation took 1hour.

EXAMPLE 38

The test was conducted as in Example 35 and with the same amounts ofmaterial but at a temperature of +40° C. (±2°) instead of 60° to 65° C.The hydrogenation process was terminated after 7 hours.

EXAMPLE 39

The test was conducted as in Example 35 and with the same amounts ofmaterial but with the concentration of MgCl₂ in the THF solutiondecreased to 0.35 moles/l (130 mmoles of MgCl₂ ; anthracene:MgCl₂=1:6.5). Hydrogenation took 4.7 hours.

EXAMPLE 40

The test was conducted as in Example 35 and with the same amounts ofmaterial but at a concentration of 0.27 mole/l of MgCl₂ in THF (100mmoles of MgCl₂ ; anthracene:MgCl₂ =1:5). Hydrogenation took 7.5 hours.

EXAMPLE 41

The test was conducted as in Example 35 and with the same amounts ofmaterial although CrCl₃ (3.2 g, 20.0 mmoles) was employed as a catalystcomponent instead of TiCl₄ 2 THF. Hydrogenation took 2 hours.

EXAMPLE 42

The test was conducted as in Example 35 and with the same amounts ofmaterial although MgBr₂ was employed in a concentration of 0.25 moles/l(90 mmoles of MgBr₂ ; anthracene:MgBr₂ =1:4.6) as a catalyst componentinstead of MgCl₂. Hydrogenation took 12 hours.

EXAMPLE 43

55.5 g (2.2 moles) of magnesium powder (50 mesh) was placed in 370 ml ofabsolute THF. 16.4 ml (0.2 moles) of 1,2-dichloroethane was dropped intothe suspension while it was being stirred, with a lot of ethylenedeveloping. The batch was stirred for 15 minutes, the dissolved ethylenepumped off, and the suspension treated with 3.50 g (20 mmoles) ofanthracene. Subsequent to 5 to 10 minutes of stirring the suspensionturned dark blue. After 15 minutes of stirring it was treated with 6.7 g(20 mmoles) of TiCl₄ 2 THF and processed further as described withreference to Example 1. Hydrogenation took 4 hours. The yield andproperties of the magnesium hydride are identical to those mentionedwith reference to Example 35.

EXAMPLE 44

The test was conducted as in Example 35 and with the same amounts ofmaterial but at a hydrogen pressure of 40 bar. Hydrogenation took 5.2hours.

EXAMPLE 45

The test was conducted as in Example 35 and with the same amounts ofmaterial but at a hydrogen pressure of 20 bar. Hydrogenation took 9hours.

It is understood that the specification and examples are illustrativebut not limitative of the present invention and that other embodimentswithin the spirit and scope of the invention will suggest themselves tothose skilled in the art.

I claim:
 1. A process for the production of magnesium hydride whichcomprises reacting magnesium with hydrogen in an organic solvent in thepresence of anthracene and a halide of a transition metal of the IVth toVIIIth sub group of the periodic system.
 2. A process according to claim1, wherein the reaction is carried out under a pressure from about 1 to300 bar.
 3. A process according to claim 1, wherein the transition metalhalide is a halide of titanium, iron and/or chromium.
 4. A processaccording to claim 1, wherein the solvent comprises tetrahydrofuran,toluene, 1,2-dimethoxyethane, or mixtures thereof.
 5. A processaccording to claim 1, wherein a magnesium halide is added to the solventand is present during the reaction with hydrogen.
 6. A process accordingto claim 1, including the further steps of dehydrogenating the resultingmagnesium hydride at elevated temperature thereby to produce activatedmagnesium, and contacting the activated magnesium with ahydrogen-containing gas to re-form magnesium hydride.
 7. A processaccording to claim 6, wherein the hydrogen is employed as a gaseousmixture with at least one other gas which is inert under the prevailingconditions.
 8. A process according to claim 6, wherein the hydrogen isemployed as a gaseous mixture with at least one member selected from thegroup consisting of nitrogen, methane, carbon monoxide and ammonia.
 9. Aprocess according to claim 1, wherein the solvent comprisestetrahydrofuran, the reaction is carried out a pressure of at most 140bar and a temperature of about 20° to 80° C., and the ratio of eachtransition metal halide and of anthracene to magnesium 1:10 to 1:10⁴.